The objective was to study the events that regulate or interfere with the thermal conductivity of neoplastic tissues and to assess the differences in local perfusion of tumors. The thermal conductivity of Walker carcinoma was found to be 2.8 plus or minus 0.8 MW/cm/degrees C under physiological conditions. The thermal conductivity varied proportionally to the square root of the perfusion rate when the blood supply to the tumor was doubled or reduced to one-half. The tumor preparation necessary to simultaneously measure temperatures in different sections of the tumor has been developed and the procedure to evaluate changes due to sudden variations in blood temperature has been completed. Determinations aimed at evaluating the loco-regional irroration of the neoplastic tissue are ongoing.